For the study, published in the British Medical Journal, falls in breast cancer death rates were compared in three pairs of similar territories: Northern Ireland to the Republic of Ireland; Sweden to Norway; and The Netherlands to Flanders in Belgium.

In the first of each pair screening was introduced 10 to 15 years earlier than in the second.

However, they found little difference between the pairs in breast cancer mortality rate falls from 1989 to 2006.

The similarities "suggest that screening did not play a direct part in the reductions in breast cancer mortality", they concluded.

Their study added to others which "found that mammography screening by itself has little detectable impact on mortality due to breast cancer".

Improvements in treatment and healthcare systems "may be more plausible explanations" for death rate falls, they found.

Dr Karsten Juhl Jørgensen, of the Nordic Cochrane Centre in Denmark, said the paper drew similar conclusions to one he had co-authored with colleague Prof Peter Gøtzsche, published in the BMJ last year.

It found breast cancer death rates fell slower from 1997 to 2006 in areas of Denmark where screening had been introduced in 1991.

He said it was "becoming increasingly clear" that those who deserved the credit for falling death rates "are those who treat breast cancer, not those who screen healthy women".

Last year the same Nordic Cochrane Centre team claimed that 7,000 women a year in Britain were undergoing unnecessary treatment - including mastectomies - after wrongly being told they had life-threatening breast cancer.

The problem is that mammograms detect both tumours and precancerous legions in milk ducts called ductal carcinoma in situ (DCIS). If left untreated half DCISs will develop into cancer while half will not.

Dr Jørgensen said: "We are turning thousands of healthy women into breast cancer patients, who will be worried about a non-disease returning for the rest of their lives. That's something we should be avoiding."

However, Dr Anna Gavin, one of the authors of today's study, and director of the Northern Ireland Cancer Registry, said she still had confidence in screening.

"If someone is invited, they should go," she said.

"But we do need to question screening and be sure that the benefits outweigh the risks."

Prof Julietta Patnick CBE, director of the NHS Cancer Screening Programmes, said: "The best evidence available shows that women aged 50 to 69 who are regularly screened are less likely to die from breast cancer.

"The World Health Organisation's International Agency for Research on Cancer (IARC) estimates that there is a 35 per cent reduction in mortality from breast cancer among screened women aged 50 to 69."